In phonetics, a vowel is a sound in spoken language that is characterized by an open configuration of the vocal tract so that there is no build-up of air pressure above the glottis. This contrasts with consonants, which are characterized by a constriction or closure at one or more points along the vocal tract. A vowel is also understood to be syllabic: an equivalent open but non-syllabic sound is called a semivowel.
In all languages, vowels form the nucleus or peak of syllables, whereas consonants form the onset and (in languages which have them) coda. However, some languages also allow other sounds to form the nucleus of a syllable, such as the syllabic l in the English word table [ˈteɪ.bl̩] (the stroke under the l indicates that it is syllabic; the dot separates syllables), or the r in the Czech word vrba [vr̩.ba] "willow".